1. Field of the Invention
The present invention relates to a distributed Bragg reflector (DBR) structure, a method of manufacturing the same, and a vertical cavity surface emitting laser (VCSEL) diode with the DBR structure. More particularly, the present invention relates to a DBR structure having an InAlAs/InP/InAlAs layer stacked on an InAlGaAs layer in a VCSEL diode, a method of manufacturing the DBR structure, and a VCSEL diode with the DBR structure.
2. Discussion of Related Art
Semiconductor laser diodes, in general, include edge emitting laser diodes and VCSEL diodes. An edge emitting laser diode has a cavity structure parallel to a stack surface of a device and emits a laser beam parallel to the stack surface. A VCSEL diode has a cavity structure perpendicular to a stack surface of the device and emits a laser beam perpendicular to the device's stack surface.
VCSEL diodes are widely used in optical communications, optical data writing, and holographic memories because they have a low driving current and a low beam divergence compared to edge emitting laser diodes. VCSEL diodes can oscillate in a single longitudinal mode and can be easily manufactured and tested in a wafer state, resulting in superior characteristics compared to edge emitting laser diodes. Additionally, since VCSEL diodes are capable of high-speed modulation and can emit a circular beam, they are easily coupled to an optical fiber and can be arranged in a two-dimensional array.
Meanwhile, a semiconductor laser diode has three necessary components for laser oscillation by stimulated emission: (i) an optically active layer, (ii) a cavity region, and (iii) mirrors. In particular, a VCSEL diode has a high reflectivity DBR structure that functions as a mirror at both sides of the cavity region having the optically active layer.
Usually, a DBR structure applicable in a VCSEL diode consists of two material layers with different refractive indices, and the material layers are alternately stacked to have a thickness of ¼ of the laser wavelength. In order to obtain high reflectivity, there should be a large difference between the refractive indices of the two materials. Therefore, GaAs and AlAs which have quite different refractive indices and are capable of being lattice-matched are widely used. A VCSEL diode manufactured using GaAs is adequately employed in a short wavelength (1 μm or less) laser diode.
Also, a method of manufacturing an optical communication device for medium and long distance transmission that emits a long wavelength (for example, 1.3 μm or 1.55 μm) beam having minimal loss in an optical fiber has been suggested. E. Hall et al. have suggested a VCSEL diode having a bandwidth of 1.55 μm using an AlAsSb/InAlGaAs DBR structure and an AlAsSb/AlGaAsSb DBR structure, which are based on Sb and have a large refractive index difference of about 0.43-0.44 (E. hall et al., Electron. Lett. 35, 425 (1999) and Electron. Lett. 36, 1465 (2000)). However, in the case of manufacturing a Sb-based laser diode, there are disadvantages such as complexity in growing the material layers forming a DBR structure, low thermal conductivity, and reduced device reliability.
In addition, a laser diode manufactured on an InP substrate using InP/InGaAsP and InAlGaAs/InAlAs has been suggested. The diode has a relatively small refractive index difference between the two materials (0.27 and 0.3) and thus does not easily yield high reflectivity. Also, if stack periods of InAlAs/InAlGaAs are repeated, defects are formed on the DBR structure's surface as its thickness increases, resulting in deterioration of device quality. Therefore, recently, an InP-induced InAlGaAs/InP DBR structure that has a relatively large refractive index difference (0.34) compared to InP/InGaAsP and InAlGaAs/InAlAs, and good thermal conductivity, has been suggested. However, when a thin film is grown by combining these two materials, an As carry-over effect occurs after growth of the InAlGaAs layer and prior to growth of the InP layer, a thin film growth interface between the two materials is not even, and thus a transitional layer such as an InAsP layer may be produced. Therefore, a complicated growth interruption is used to manufacture an InAlGaAs/InP DBR structure having a high quality epitaxial characteristic (T. C. Lu et al., Journal of Crystal Growth., 250, 305 (2003); T. C. Lu et al., Material Sci. & Eng. B107, 66 (2004)).
Below, an InAlGaAs/InP DBR structure forming a VCSEL diode and problems associated with its composition will be described in detail with reference to the drawings.
FIG. 1 is a schematic side cross-sectional view (a) of a conventional vertical cavity surface emitting laser (VCSEL) diode including an InAlGaAs/InP DBR structure, and a diagram (b) illustrating a vertical cavity surface emitting energy band of the VCSEL diode.
Referring to FIG. 1, a conventional VCSEL diode 100 includes an InP substrate 110, a lower DBR structure 120 disposed on the InP substrate 110, a cavity 130 disposed on the lower DBR structure 120, and an upper DBR structure 140 disposed on the cavity 130.
The cavity 130 includes a pair of InP layers 131 serving as a guide layer and a clad layer of an optical field, and an active layer 132 interposed between the InP layers 131. The active layer 132 includes a pair of barrier layers 133 contacting the InP layers 131 and a quantum well layer 134 interposed between the barrier layers 133. The depth (d) of the cavity 130, if a laser wavelength is denoted by λ, generally is d=(k+½)×λ (k=an integer). The quantum well layer 134 of the cavity 130 formed as mentioned above may have a maximum electric field strength. The cavity 130 of FIG. 1 is just an example, and various structures such as a tunnel junction have recently come under study.
The DBR structures 120 and 140 disposed on and under the cavity 130 have a form of alternating stacks of InAlGaAs layers 121 and 141 and InP layers 122 and 142. In general, if a wavelength is λ, and refractive indices of two materials are n1 and n2 respectively, thicknesses of the InAlGaAs layers 121 and 141 and InP layers 122 and 142 are λ/4n1 and λ/4n2, respectively. When the DBR structures are manufactured with a combination of the InAlGaAs layers 121 and 141 and InP layers 122 and 142, compositions of the InAlGaAs layers 121 and 141 should have the largest possible difference in refractive index from the InP layers 122 and 142, and energy band gaps of the InAlGaAs layers 121 and 141 should be larger in order not to absorb light at the wavelength of the active layer 132.
In the InAlGaAs/InP DBR structures 120 and 140, GaAs and AlAs are lattice-matched, and In0.52Ga0.48As is lattice-matched to the InP substrate. For this reason, in the InAlGaAs material, when a composition ratio of [Ga+Al] is selectively changed within about 48% depending on usage while a composition ratio of In is about 52%, the InAlGaAs material can be lattice-matched to the InP substrate. In the case that the DBR structures 120 and 140 are employed in a VCSEL diode having a wavelength of 1.55 μm, as the composition ratio of Ga in In0.52AlGaAs increases, the photoluminescence wavelength becomes longer and the refractive index difference from InP increases while the lattice-matching to InP is maintained. So, there is no difficulty in adopting an InAlGaAs composition. Consequently, with the InAlGaAs 121/InP 122 DBR structure, high reflectivity can be obtained even with only about 35 stack periods.
However, because wavelength and refractive index are inversely proportional, in the case of employing the DBR structure having the above composition in a VCSEL diode having a wavelength of 1.3 μm, it is not easy to set the composition of the InAlGaAs material. For example, when InAlGaAs having a wavelength of about 1.2 μm, which does not overlap a wavelength of 1.3 μm, is chosen, the InAlGaAs is joined to InP in a band line-up of type II, not type I. For this reason, a transition wavelength may broadly range from 1.28 to 1.3 μm, thereby overlapping the 1.3 μm laser wavelength. In this case, a photon produced from the active layer of the VCSEL diode having a laser oscillation wavelength of 1.3 μm round-trips in the multiple-period DBR layer without reflection gain. In result, the photon is taken by an absorption band formed by the type-II band line-up, and thus overall performance of the laser diode deteriorates.
Referring to (b) of FIG. 1, type-II band line-up is formed in the in AlGaAs/InP DBR structures 120 and 140. In the case that the InAlGaAs/InP DBR structure 120 is employed in the VCSEL diode using a wavelength of 1.3 μm, the stacking period of InAlGaAs/InP should be increased in order not to form an absorption band corresponding to around the laser oscillation wavelength in the DBR layer. Also, selecting InAlGaAs, which has a relatively short wavelength, requires that the number of stack periods of the DBR structure be increased to obtain high reflectivity.
For example, InP (refractive index: about 3.2) and InAlGaAs (refractive index: about 3.4) used in the VCSEL diode oscillating at 1.3 μm have thicknesses of about 101.6 nm and about 94 nm, respectively. Thus, there should be at least 48 stack periods in order to obtain a high reflectivity of at least about 99.5%. As such, since increasing the stack periods may cause deterioration of properties of the device due to formation of defects on its surface, a composition of InAlGaAs having a wavelength as close as possible to an oscillation wavelength should be selected to reduce the number of stack periods of the DBR structure.
FIG. 2A is a graph plotting photoluminescence intensity and reflectivity according to a wavelength of an InAlGaAs/InP DBR in accordance with a conventional art. And, FIG. 2B is a diagram illustrating emission/absorption transition occurring due to type-II band line-up in a conventional InAlGaAs/InP junction. Considering only compositions of InAlGaAs layers 121 and 141 having a wavelength of 1.2 μm, since a photoluminescence (PL) wavelength is 1.2 μm, reflectivity should not be affected. However, an actual PL wavelength produced by forming a junction with InP layers 122 and 142 is 1.29 μm because of type-II transition, as shown in FIG. 2B. Accordingly, in a PL wavelength region, a stop-band of reflectivity is not maintained at a high level of reflectivity and reflectivity drops to form a deep. Consequently, photons produced from an active layer 132 of a 1.3 μm VCSEL diode are absorbed while traveling through the DBR structure, such that overall efficiency of the laser diode deteriorates and the wavelength of the laser changes depending on the temperature, which can have a critical, adverse effect on characteristics of the device.